Internal combustion engines can be used to power a generator and create electric energy which is stored and in turn drives an electric motor to propel a vehicle. This technique has been used effectively in vehicles such as locomotives and hybrid automobiles. Typically this process involves a generator that produces AC power. The AC power can be converted to DC power using, in some cases using a rectifier, but more popularly uses an AC-DC converter. The DC power can then be stored and a DC-AC inverter can be used to supply 3-phase power to an AC motor used to drive the vehicle.
This arrangement, while relatively efficient, has several drawbacks. One involves the cost of the high power components used in the AC-DC converter and the DC-AC inverter. Another is the loss of inertia provided by the flywheel of a standard engine-clutch-transmission drivetrain that allows smoother performance in the event of momentary engine power changes.
WO0034066 (“the '066 patent”) discloses a dual rotor machine with a stator surrounding an output rotor and an input rotor surrounded by the output rotor. The stator and input rotor each have windings, the output rotor has embedded permanent magnets that interact with fields generated at the stator and input rotor. The input rotor requires slip rings to carry current to the input rotor windings. The permanent magnets of the '066 patent impose a substantial cost penalty on the system. The slip rings of the '066 patent also have a cost impact due to the elaborate construction requirements and also create a reliability weakness at the contacts between the slip rings and the shaft.